1. Field of the Invention
The present invention relates generally to thermoelectric cooling, and more particularly to a system and its methods of use for cooling an interior of a vehicle by using a thermoelectric cooling assembly powered a solar photo-voltaic panel.
2. Description of the Related Art
Interior compartments of stationary vehicles tend to get enormously hot particularly during sunny, daylight periods of the day. On a warm sunny day, for example, a vehicle's windows collect light, trapping heat inside the vehicle and pushing the temperature inside to dangerous levels (100-130° F.). Such temperature increases can occur in a car even if the windows are opened slightly. An extremely hot interior of a vehicle poses a danger to pets, electronics and heat-sensitive items, such as medications, left in the vehicle, such as a car. An extremely hot stationary vehicle interior increases the cooling load on the vehicle air-conditioning unit when the vehicle is operated.
An existing method in cars to passively prevent this heating problem involves aluminum-coated reflectors mounted on the windshields of the parked cars which reflect the sun's incoming radiation. This technique is not effective because these reflectors are not so efficient in preventing the sun's radiation from entering the car and eventually the temperature inside the car becomes undesirable.
A thermoelectric cooler (TEC), also known as a thermoelectric module or Peltier cooler, is a semiconductor-based electronic component that functions as a small heat pump. By applying a low voltage DC power source to a TEC, heat will be moved through the thermoelectric material from one side to the other. One cooler face, the cold side, therefore is cooled while the opposite face, the hot side, is simultaneously heated.
FIG. 1 is a diagram of a practical TEC 101 comprising two or more elements of p-type and n-type semiconductor material P and N that are connected electrically in series and thermally in parallel. The semiconductor material is generally bismuth telluride. The elements of semiconductor material P and N are biased by a low DC voltage provided by a DC power source 102. These thermoelectric elements and their electrical interconnects typically are mounted between two ceramic substrates 103 and 104. One ceramic substrate is the cold side 103 removing heat from an object being cooled 105. The object being cooled 105 may, in turn, be used to remove heat from another object or air. A heat sink 106 must remove from the other ceramic substrate, the hot side 104. In turn, heat must be removed from the heat sink 106. The heat sink may have fins fabricated into to enhance the exchange of heat between it and air and/or water. Placing TEC's on top of one another in stages to form a multi-stage thermoelectric module increases cooling performance.
Like mechanical refrigerators, TEC's are governed by the same fundamental laws of thermodynamics. In a mechanical refrigeration unit, a compressor raises the pressure of a liquid and circulates the refrigerant through the system. In the evaporator or “freezer” area, the refrigerant boils and in the process of changing to a vapor, the refrigerant absorbs heat causing the freezer to become cold. The heat absorbed in the freezer area is moved to the condenser where it is transferred to the environment from the condensing refrigerant.
In a thermoelectric cooling system, a doped semiconductor material essentially takes the place of the liquid refrigerant, the condenser is replaced by a heat sink 106, and the compressor is replaced by a DC power source 102. The heat sink 106 may be fabricated with fins to exchange heat from the heat sink 106 with surrounding air. The application of DC voltage to the thermoelectric module causes electrons to move through the semiconductor material. At the cold side 103, heat is absorbed by the electron movement, moved through the semiconductor material P and N, and expelled at the hot side 104.
It should be noted that thermoelectric modules can only transfer heat from the cold side 103 to the hot side 104, but cannot dissipate heat by themselves into the atmosphere. Hence, heat sinks must be in contact at the hot side 104 of the thermoelectric module to dissipate heat to the atmosphere through convection. Applications for thermoelectric modules cover a wide spectrum of product areas. These include equipment used by the military, medical, industrial, consumer, scientific and telecommunication organizations. Uses range from simple food and beverage coolers for an afternoon picnic to extremely sophisticated temperature control systems in missiles and space vehicles. Some of the more significant features of thermoelectric modules include: no moving parts, small size, ability to cool below ambient as well as heat above ambient, reliability and environmental friendliness.
FIG. 2 is a diagram of the top view of a small enclosure 200 air-conditioned by the thermoelectric effect. An air-to-air heat exchanger 201 is used for the cooling of air in an enclosure. The air-to-air heat exchanger 201 utilizes the thermoelectric effect whereby the heat is transferred via the flow of current through thermoelectric modules 202. One part absorbs the heat and, as a consequence, reduces the temperature on the cold side 203 and the other part dissipates the heat to ambient on the hot side 204.
Fans 205 and 206 are used to move air over heat sinks 207 and 208 on both the hot and cold sides of the thermoelectric modules. The cold side 203 of the modules 202 is connected to a heat sink 207 with a fan 205 (forced convection) that absorbs heat from within the enclosure 200 and circulates the cooled air. The hot side 204 of the thermoelectric modules 201 is connected to another forced convection heat sink 208 that dissipates the heat absorbed through the cold side 203 to the atmosphere. Forced convection improves the cooling performance. In FIG. 2, the thermoelectric modules 201 form the active cooling element and the fans 205 and 206 in combination with the respective heat sinks 207 and 208 form the passive cooling elements.
U.S. Pat. No. 6,119,463 describes a thermoelectric cooling system that cools seats by thermoelectric cooling air supplied to passages inside the surface of a seat. The heat is removed from the hot side of the TECs by a heat exchanger cooled by air passing over it and into the interior compartment of the vehicle. Thus, this apparatus cools the seat surface but heats the air in the interior of the vehicle.
Solar photovoltaic panels have been permanently constructed into vehicles. In land vehicles, such as cars and trucks, these panels have been built in the sun roof to supplement electric power for various applications in the vehicle. Some of these applications can operate without use of electric power provided by the vehicle's engine.
Permanently installed, solar photovoltaic panel-powered thermoelectric cooling systems have been developed for cars. Thermoelectric car air-conditioning has been previously described. Japanese Patent Application Publication No. 08-011517 discloses a built-in thermoelectric air-conditioning apparatus for a car powered by the battery, in turn, powered by the engine or, alternatively, a solar panel installed on top of the roof. However, this apparatus must be factory-installed for new cars or retrofit, at significant expense, in existing cars. Because the apparatus sticks out of the floor of the car, it is intrusive to the driver and/or passenger.
Additionally, Japanese Patent Application Publication No. 11-342731 discloses solar photovoltaic panel-powered thermoelectric cooling system. However, this system must also be factory-installed for new cars or retrofitted, at significant expense, in existing cars. Additionally, in a parked state, the hot sides of the TEC's are inefficiently passively cooled by external air flowing in a narrow passage between roof of the car where the TEC's are installed and the bottom of the solar photovoltaic panel. The apparatus sticks out of the roof of the passenger automobile such that the aerodynamics, stability and structural integrity of the car are compromised.
Accordingly, there is a need for a need for a compact, removable apparatus to prevent the temperature inside a vehicle from becoming dangerously high during stationary periods in sunny conditions without requiring an expensive retrofitting of existing vehicles, compromising the structures of vehicles, or using the battery of the vehicle.